Expression of Axl family members and their ligands in human CD34⁺ HPCs. Isolated CD34⁺cells from human peripheral blood were cultured in the presence of IL-15 for 0, 5, 10, or 15 days. Then nonquantitative assessment of Gas6, Axl, Sky (Tyro3), Mer, protein S, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA expression was performed by RT-PCR. These results are representative of 2 experiments.